This invention relates generally to vidicon-type camera tubes, and more particularly to an improved semiconductor target for use in such camera tubes.
Semiconductor targets used in vidicon type camera tubes typically comprise an n-type semiconductor crystal substrate having an array of light-sensitive elements arranged on one of its principal surfaces. The camera tube operates in a storage mode utilizing the depletion region formed by the reverse biasing of the individual light-sensitive elements. Accordingly, the dark current of vidicon tube targets is a recombination current that is thermally generated at recombination centers in the depletion region. Almost all of the recombination centers are in the interface-state (fast-state) at the interface between the n-type semiconductor crystal substrate and the insulating layer formed on the surface thereof for passivating the substrate surface regions lying between the light-sensitive elements. The magnitude of such target dark current is thus largely dependent on the fast-state density in the depletion region.
It is also known that the dynamic range of a vidicon tube depends largely upon the magnitude of the target dark current. For instance, in an ordinary mode of operation, a vidicon tube having a dynamic range of 32 dB with a dark current of 10 nA exhibits a dynamic range of 38 dB with a dark current of 5 nA and of 46 dB with a dark current of 2 nA. That is, the dynamic range of a vidicon tube increases as the dark current is reduced.
Moreover, when a vidicon tube is operated for an extended period of time, soft x-rays are generated therein and irradiation of the semiconductor target by such x-rays acts to increase the fast-state density of the target and hence its dark current. This increase in target dark current is proportional to the fourth to fifth power of the field mesh potential of the vidicon tube and to the length of time of tube operation. In other words, when a conventional vidicon tube is operated over an extended period of time there is a reduction in dynamic range, which is material, particularly in applications in which a high field mesh potential is employed to improve image resolution.
Under these circumstances, it is necessary to minimize the initial value of dark current while, on the other hand, preventing any substantial increase in dark current produced by irradiation of soft x-rays generated in the vidicon tube in order to obtain a dynamic range of a substantial width and to prevent reduction in the width of the dynamic range.
It has been suggested that an increase in dark current can be nearly completely prevented by forming a vapor-deposited film of PbO on the entire electron beam scanned surface of the semiconductor target as a semi-insulating layer thereon. This film is intended to reduce the negative charge buildup caused by scanning electrons on the surface of the insulating layer formed on the target. However, in the operation of semiconductor targets having a PbO film vapor deposited directly on their electron beam scanned surface, a partial extreme rise in dark current value must occur which is observed as a white blur on the dark current pattern of the vidicon tube, as such targets are heated unevenly during the vapor deposition of PbO and after their insertion into tube envelopes in different thermal stages of tube fabrication ending in the stage of vacuum sealing.